With the advent of idling-stop vehicles, lead-acid batteries have been deeply discharged more often than before. For example, lead-acid batteries of idling-stop vehicles are based on the premise that they are used in a partial state of charge (PSOC). Lead-acid batteries for cycle applications like those for forklift trucks have been used at a deep depth of discharge (DOD). When a lead-acid battery is used in a partial state of charge, its life is reduced due to accumulation of lead sulfate in a positive electrode or sulfation of a negative electrode. In the partial state of charge, agitation of an electrolyte solution by gas evolution is insufficient, and thus the electrolyte solution is easily stratified. This further reduces the life of a lead-acid battery.
Meanwhile, when the lead-acid battery transitions into an overdischarged state from the partial state of charge because, for example, the vehicle is left unattended for a long period of time, metal lead passes through the separator, and thus a permeation short circuit, in which both positive and negative electrode plates are short-circuited, easily occurs. The concentration of sulfate ions in the electrolyte solution decreases due to overdischarge, and correspondingly, the concentration of lead ions in the electrolyte solution increases. The lead ions are reduced on the negative electrode plate during charge, and metal lead dendrite grows through pores inside the separator, and thus passes through the separator to short-circuit the positive electrode plate and the negative electrode plate.
The applicant proposed improvement of the life of a lead-acid battery in the PSOC by including graphite in a negative electrode material. For example, Patent Document 1 (WO 2011/90113) discloses that a negative electrode material is made to contain 0.02 to 2.20 mass % of graphite, 0.5 mass % of barium sulfate, and 0.02 to 2.20 mass % of carbon black. Patent Document 2 (WO 2011/52438) discloses that a negative electrode material is made to contain 0.5 to 3.0 mass % of expanded graphite and 0.6 mass % of barium sulfate. Among documents other than those by the applicant, for example, Patent Document 3 (JP5584216 B) discloses that a negative electrode material is made to contain 1 to 3 mass % of graphite, 0.8 mass % of barium sulfate, and 0.1 to 2 mass % of carbon black.